The present invention relates to electric drive systems for use in electric vehicles, tractors, etc . . . and, more particularly, to an improved electric motor and driver system that includes a unique stator construction which provides on-command variable torque control at any constant revolution per minute (xe2x80x9cRPMxe2x80x9d) and, thereby, improves motor efficiency that results from reduced power consumption and increased vehicle mileage.
As is generally known in the art, electric drive systems in recent years have been limited to small vehicles with a direct current (DC), an alternating current (AC) or a brushless direct current (DC) electric motor with a conventional stator winding construction that is connected through a speed control unit to a battery source. All of these conventional motors have fixed turns per coil stator windings and all of the stator windings are wired in a fixed electrical configuration to the switching circuits and the power source.
In an electric drive system for vehicles, the motor must be capable of generating a relatively high amount of torque to accelerate the vehicle. Once the vehicle reaches a constant speed, the momentum of the vehicle will reduce the torque load on the motor to a low torque level which is limited to the amount of torque that is required to overcome the aerodynamics, road resistance, and mechanical losses of the vehicle. As the conditions such as wind and terrain fluctuate, the amount of torque load on the motor required to maintain the constant speed of the vehicle will also fluctuate. To maintain a constant vehicle speed, the motor must also maintain a constant speed while encountering these major changes in torque load. These fluctuations in the torque load, however, while maintaining a constant RPM, result in a reduced motor efficiency causing problems such as excessive heat in the drive system, wasted energy, and reduced vehicle mileage.
The efficiency problems identified above are presented by state of the art electric drive systems with conventional motors that have fixed turns per coil stators. By way of illustration, FIGS. 1 and 2 graphically illustrate these efficiency problems for a conventional AC induction drive system.
FIG. 1 represents system power efficiency curves in relation to torque load and RPM of a conventional AC induction drive system. At one hundred percent (100%) torque load, the peak motor efficiency is low and the RPM range is small. Specifically, the peak motor efficiency is at approximately seventy-three percent (73%) at approximately 1700 RPM. As the torque load is reduced by one half (xc2xd) to fifty percent (50%), the peak motor efficiency will increase slightly to approximately seventy-nine percent (79%) and the operating RPM range will more than double to approximately 3600 RPM. As the torque load is reduced by another one half (xc2xd) to twenty-five percent (25%), the peak motor efficiency will continue to increase to approximately eighty-four percent (84%) and the operating RPM range will increase almost one and a half times (1xc2xd) to approximately 5800 RPM. As the torque load is reduced by another one half (xc2xd) to twelve and one-half percent (12.5%), the peak motor efficiency will continue to increase to approximately ninety percent (90%) and the operating RPM range will increase another one and one quarter times (1xc2xc) to approximately 7200 RPM. As the torque load is reduced below ten percent (10%), however, the peak motor efficiency will decrease to approximately eighty-one percent (81%) while the operating RPM range will remain substantially the same at approximately 7200 RPM. Thus, as the torque load is reduced, the peak motor efficiency generally increases and the operating RPM generally increases substantially. Conversely, as the torque load is doubled, the peak motor efficiency generally decreases and the operating RPM will be reduced by fifty percent (50%).
FIG. 2 represents the amount of horsepower a typical direct drive vehicle might require to maintain a constant mile per hour (xe2x80x9cMPHxe2x80x9d) speed. As illustrated, the greater the MPH and operating RPM of the vehicle, the greater the horsepower to maintain the constant speed and operating RPM of the vehicle. For example, at a constant 45 MPH, the vehicle would require approximately seven horsepower (HP) which is less than twenty percent (20%) torque load at approximately 3600 RPM. In FIG. 1, a twenty percent (20%) torque load at 3600 RPM would result in a power efficiency of approximately eighty-five percent (85%). However, to accelerate this vehicle to pass another vehicle, the torque load may double. This could cause the efficiency to drop five percent (5%) to fifteen percent (15%). This is another example of low efficiency in electric motors with fixed turns per coil stator windings.
Also, FIGS. 1 and 2 show that a constant speed of 20 MPH may require only 4 HP. This is below the ten percent (10%) torque load level and the motor efficiency could again drop five percent (5%) to fifteen percent (15%). Another problem is when the vehicle is accelerated from zero MPH, the high torque at very low RPM levels could result in the one hundred percent (100%) torque load range and motor efficiency in the sixty percent (60%) range. Anytime the efficiency drops below eighty-five percent (85%), the vehicle mileage range is reduced drastically because this energy is wasted in excess heat in the drive system.
In an electric drive vehicle, the system power is equal to the current squared times the circuit resistance in the motor or, in the equation of, Power=(Current2)(Resistance). Since battery voltage in an electric drive vehicle is relatively constant, if the current is decreased, the power consumption of the motor is reduced which increases the vehicle mileage. For example, if the current is decreased by approximately twenty-five percent (25%), the corresponding reduction in power consumption can result in an increase of vehicle mileage of approximately fifty percent (50%) to one hundred percent (100%). However, for fixed turns per coil stator windings, as illustrated in FIG. 1, a reduction in torque load below ten percent (10%) results in a less efficient motor which will instead decrease the vehicle mileage.
Based upon the foregoing, the state of the art electric drive systems present a number of problems. First, fixed turns per coil stator windings cannot compensate for low efficiency conditions. As a result, power consumption is increased which ultimately reduces the mileage that the vehicle may operate on the battery source. Second, as the power consumption is increased and corresponding vehicle mileage is reduced, the vehicle requires more frequent recharging or replacement of the battery source for the vehicle to continue in operation. Such inefficiencies result in larger battery source requirements or larger hybrid generators. Lastly, the inefficiency of the drive systems present a major obstacle to the manufacturers in the development and marketability of electric drive vehicles such as passenger cars, trucks, tractors, etc . . . to the consuming public.
Thus, there is a need and there has never been disclosed the improved electric motor and driver system with a unique stator construction that enables variable torque control at constant RPM to improve motor efficiency.
It is the primary object of the present invention to provide an improved electric motor and drive system to control the amount of generated torque at any constant speed to significantly increase motor efficiency and reduce power consumption.
Another object of the present invention is to provide an improved electric motor which includes a unique stator construction which provides that each pole within each phase will include at least two split windings. A related object of the present invention is that when the poles within each phase are combined, the magnetic fields from all split windings within each pole will add to one another.
Another object of the present invention to provide an improved electric motor which includes a unique stator construction where each split winding in all poles and phases be wired into a partitioned wiring configuration.
Still another object of the present invention to provide an improved electric motor which includes a unique stator winding connection where each partition of split windings are independent and connected to H-bridge drivers.
It is another object of the present invention to provide an improved electric drive system where the power supply inputs to each partition having switching circuits that can switch the supply power to the partition drivers into parallel or series configurations with respect to the other partition drivers so as to effectively change the stator winding turns per coil.
Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying drawings.
The present invention is an electric motor that can generate variable levels of torque at any constant RPM to a driven member over a wide range of RPM speeds so as to reduce power consumption to the power source. The motor includes a rotor having an axis of rotation and a stator assembly which is disposed around the rotor and is inductively coupled to the rotor. The stator assembly has a plurality of poles with each of the stator poles in each phase consisting of at least two split windings.
The split windings within each of the stator poles and phases are wound to be essentially in phase with each other so that the magnetic flux generated in each of the stator windings will add to each other to produce an increased magnetic field strength or, in other words, as each of the split winding partitions are switched into different parallel or series combinations, the turns per coil is effectively changed resulting in a different torque efficiency curve.
A micro-processor controller is used to input the phase currents and voltage, RPM of the motor, the MPH of the vehicle, the throttle level and other necessary signals to determine the configuration of the split partitions required to achieve maximum efficiency at the current torque load in addition to standard Pulse-Width-Modulation functions.